This multidisciplinary program of research deals with the ultrastructural analysis of the fetal and neonatal brains with specific emphasis directed toward the cellular composition and organization of the median eminence, neural lobe and other circumventricular organ systems (CUOs). This study will employ correlative scanning-transmission (STEM) electron microscopy, light and electron microscopic autoradiography to assess the neuroanatomical organization and distribution of 3H dopamine in the fetal, neonatal and adult rat brain. In addition we will employ a new approach that utilizes implanted normal fetal hypothalamic tissue into the third cerebral ventricle of adult host rats with autosomal diabetes insipidus. Morphological analysis of the fetal explants, fetal-host interaction(s) and the physiological assessment of recovered function in magnocellular system of the Brattleboro rat may shed more light upon hypothalamic systems which synthesize centrally active peptides. Stem analysis of the CVOs of aged rat brains have also been undertaken to assess the impact of aging upon the blood brain - blood-CSF barrier in these regions of neuroendocrine transduction.